galactic halo

A spherical region around a spiral galaxy. The halo round our own Galaxy extends to a radius of about 50,000 light years. It is the region within which all the stars belonging to the galaxy are found, particularly the ancient globular clusters. The halo contains very hot gas that emits X-rays. The stars outside the disc of the galaxy but within the halo are the oldest, and record the size of the galaxy before much of it collapsed to a disc.